The present invention relates generally to nanolithography, and more specifically the invention pertains to a fabrication efficient method of designing multipassband grating filters.
Periodically spaced arrays are known to strongly reflect plane waves of specific temporal frequencies determined by phase matching between the wave vector and the grating period. These structures have been applied as filters in distributed feedback laser diodes, distributed Bragg reflector fiber optic filters, planar integrated optics and volume holography. In the earliest implementations of these devices, it was common to interfere two plane waves in photosensitive films such as photoresists, photographic film, or photorefractive media to produce gratings having single wavelength reflection passbands. However, a much more general range of frequency responses is available by individually setting the position and reflectivity of each reflector in a grating. For example, filters that have multiple passbands can be designed, and it even is possible to specify different levels of attenuation and bandwidth for each passband. The generalized filter functions provide important building blocks for wavelength multiplexing, demultiplexing, sorting and routing functions for fiber communications systems.
The use of atomic force microscopes in fabrication processes is disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 5,883,705, Mar. 16, 1999, Atomic force microscope for high speed imaging including integral actuator and sensor, Minne, Stephen; PA0 U.S. Pat. No. 5,880,012, Mar. 9, 1999, method for making semiconductor nanometer-scale wire using an atomic force microscope, Ha, Jeung-Sook; and PA0 U.S. Pat. No. 5,252,835, Oct. 12, 1993, Macining oxide thin-films with an atomic force microscope: pattern and object formation on the nanometer scale, Lieber, Charles.
While SPMs can provide nearly complete analog control of grating parameters, it is usually desirable if the number of fabrication variables can be reduced. This can accelerate the development, verification and, especially, the calibration of the fabrication processes. Achieving this partial control then establishes the level needed to begin developing more extensive analog control of the device parameters. Following this basic philosophy we introduce a fabrication efficient method of designing multipassband filters, as described below.